An organic light emitting diode is a light emitting element utilizing organic electroluminescence (hereinafter, organic EL), and typically include an organic EL layer sandwiched between an anode conductive layer and a cathode conductive layer. The organic EL layer includes a light emitting layer containing an organic light emitting material. As necessary, the organic EL layer includes, for example, an electron injection layer, an electron transport layer, a hole injection layer, and a hole transport layer, in addition to a light emitting layer. Organic light emitting diodes can be categorized into two types, the bottom-emission type and the top-emission type, depending on the face through which light from the light emitting layer is extracted to the outside.
Organic light emitting diodes have advantages such as smaller viewing angle dependence, less power consumption, and ability to form a very thin product. However, one problem with organic light emitting diodes is their low luminescence intensity compared with, for example, nitride semiconductor light emitting elements, and thus there is a need for improvement in light extraction efficiency. The light extraction efficiency is a ratio of the amount of light energy released to the atmosphere from the light extraction face (e.g., substrate face for the bottom-emission type) to the amount of light energy emitted from the organic EL layers. Light from the organic EL layer radiates in all directions, and much of the light propagates in a guided mode, with the total reflection repeated between the interfaces of a plurality of layers having different refractive indices. As the light propagates between layers, they may be converted to heat or be released from a side face. As a result, the light extraction efficiency is low. In addition, the distance between the organic EL layer and the cathode, which is a metal, is small. Because of this, some of the near-field light from the organic EL layer is converted into surface plasmons on the surface of the cathode conductive layer and lost. As a result, the light extraction efficiency is low:
The light extraction efficiency affects the luminance of a display or an illumination device including the organic light emitting diodes, and thus a variety of techniques for improvement have been studied. One proposed approach to improve the light extraction efficiency is a technique of utilizing surface plasmon resonance. For example, Patent Literatures 1 and 2 disclose techniques for providing a two-dimensional grating structure on the surface of the cathode conductive layer by providing the grating structure on the substrate and stacking layers thereover up to the cathode conductive layer to replicate the grating structure thereon. In the techniques of Patent Literatures 1 and 2, the two-dimensional grating structure provided on the surface of the cathode conductive layer serves as a diffraction grating. This enables extraction of energy, in the form of light, which is otherwise lost as surface plasmons on the surface of the cathode conductive layer. As a result, the light extraction efficiency is improved.